Various welding processes for the continuous production of longitudinally welded tubes are known.
Processes that are used include not only the high-frequency resistance welding or induction welding process but also the arc welding process (GTAW welding process) and the laser beam welding process. The laser beam welding process in particular has become increasingly important in recent years. Disadvantages of the laser beam welding process are the high capital costs and operating costs and the fact that the welding of metals with a bright surface, e.g., copper, causes problems, since a large portion of the laser energy is reflected by the bright metallic surface and thus is not available for the welding process. In addition, special precautions must be taken to protect personnel from misdirected laser beams.
The high-frequency welding process is characterized by high production rates, but the weld seam leaves much to be desired, so that high-frequency-welded metal tubes are unsuitable for encasing plastic tubes or electrical cables.
DE 22 56 851A describes a system for welding thin plates passing under a stationary welding head, especially for welding the longitudinal edges of a metal strip that has been shaped into a tube. The welding head of this system has three nonconsumable electrodes that are arranged in succession in the direction of the weld seam and supplied by separate current sources. The use of three electrodes serves to increase the welding travel speed, i.e., all three electrodes participate in the welding operation. If one of the electrodes becomes worn out, the welding operation must be interrupted, with the result that the production process must be stopped and started up again or that, in the case of a continuous operation, the longitudinal seam is not welded over a certain length.
The service life of an electrode in the case of arc welding under a protective gas depends on the degree of contamination of the strip material that is used. In practice, lengths of up to 1,000 m can be produced in the case of the welding of aluminum strip.